Fluidized bed reactors are commonly used for reacting a solid phase reactant with a gas phase reactant. The direct product from the fluidized bed region of such reactors, often referred to as the exit gas stream, contains both a product gas, as well as solids which contains un-reacted solid phase reactant(s) or reaction products (ashes). These solids are in the form of particles of various sizes. In order to increase the usage efficiency of the solid phase reactant, and to reduce waste ash volume, one or more stages of cyclones (multiple-stage cyclone) are often used in such fluidized bed reactors.
In a multiple-stage cyclone set up, a first stage cyclone may collect large particles and recycle the solids to the fluidized beds through a suitable dipleg with various mechanical or non-mechanical valves. Generally, however, this first stage cyclone is not sufficient or effective in removing fine solid particles. Thus, after the first stage cyclone, there is generally at least another stage cyclone to collect the fine particles escaped from the first stage cyclone. If the fluidized bed is operated at a temperature of below 800° C., it is common practice to use the cyclone dipleg insert into the fluidized bed as those in Fluid Catalytic Cracking units; see e.g. U.S. Pat. Nos. 3,652,446, 4,220,623 and. 4,578,183.
In cases where the fluidized bed is operated at above 800° C. or if oxidization atmosphere exists in the bed, it becomes difficult to use a dipleg insert into the bed. That is because of the tendency of the bed materials to form clinker inside and on the surface of the dipleg.
It is also critical to prevent the gas from reverse flowing from the fluidized bed through the cyclone dipleg to the cyclone. Fluid Catalytic Cracker (FCC) in the refinery industry, for example, uses various flapper valves on the end of the dipleg to prevent the gas from reverse flowing back into the dipleg. Flapper valves can seal the gas from reverse flowing while allow the solids to flow out of the dipleg. The dipleg with a flapper valve in the end will allow the dipleg and the cyclone to be installed inside the fluidized bed. It is prohibitively expensive, however, to use flapper valves at high temperatures, due to the need to use materials resistant to high temperatures. For a fluidized bed combustor or a gasifier, the bed temperatures can be as high as 1050° C., at which most metals would lose strength. In fact, it is practically impossible to use flapper valves.
Another method to seal the dipleg and prevent reverse gas flow is to bury the solids exit end of the dipleg inside the fluidized bed. However, elaborate designs are required to have the dipleg inserted into such a bed due to stringent requirements of the materials of construction. Therefore, it is a common practice to have the dipleg introduced to the gasifier from the outside. In this manner, the interconnection part of the solids inlet to the fluidized bed and the dipleg will be lined with refractory, which can tolerate higher temperatures.
A particular application of the fluidized bed is a fluidized bed gasifier for converting carbonaceous materials such as coal into fuel or synthesis gas. In these gasifiers, after the first stage cyclone, the particle size of the solids remaining in the gas generally is less than about 100 micron, with a mass mean diameter of about 15 to about 20 micron. These particles are generally cohesive and difficult to be fluidized, because in order to effectively return these solids back to the gasifier, it is highly desired to have the particles behave like fine sand so that they will flow like liquid; the cohesive particles from the second stage cyclone, however, behave much like wheat flour, forming lumps and many channels without being fluidized. Therefore, it is very difficult to recycle the solids back to the gasifier.
The cohesive nature of the solids creates another difficulty for solids recycling. Ordinarily, solids collected in the cyclones flows in free fall down the leg, forming a dense column in the bottom section of the dipleg. The dense phase of solids when fluidized can form a static head. which is necessary for the solids to move from the bottom of the cyclone dipleg, a relatively low pressure region, to the fluidized bed region, a relatively high pressure region. The downward flow of solids also maintains the seal in the system owing to the existence of the column of solids. The cohesive solid particles in the cyclone dipleg, however, cannot establish a static head. This is particularly true with regard to the particles collected in the secondary stage cyclone. Therefore, the solids collected by the second stage cyclone dipleg cannot be naturally recycled to the fluidized bed due to the cohesive particles in the dipleg.
Due to the above difficulties in recycling fine particle solids to the gasifier, commercial fluidized bed gasifiers generally do not use a second stage cyclone. As a result, the content of un-utilized solid reactant, for example, carbon, in the solids from the exit of the first stage cyclone cannot be utilized, resulting in low gasifier carbon conversion rate. For example, the rate of gasifier carbon conversion utilizing existing technology is only around 80-85% even with high reactive coals such as lignite and subbituminous coals. The carbon loss is a serious economic disadvantage, as well as an environmental hazard due to large amount of ash discharged. Furthermore, the carbon-containing fine solids are lighter, generally about 4-5 times higher in volume than water per unit mass, making it very difficult to handle and be further utilized. Once the carbon in the solids is more fully utilized, the final solids particles would have a higher bulk density and much easier to wet and handle.
Accordingly, there is a need for a new apparatus that can return solids from a low pressure to a high pressure region, thereby improving carbon conversion rate of a fluidized bed gasifier.